Davis plastic sextants have emerged as truly viable instruments for celestial navigation. Their accuracy is not as great as with metal sextants, and their errors vary with temperature, making frequent IC checks advisable. They make acceptable “first” sextants if finances are a constraint, and are always useful as back-up sextants. Otherwise a metal sextant will be easier to use and will give better feedback to the beginner as to how well they are doing. But once learned you can always get home with a plastic sextant.
The Mark 15 model has many great features including seven large sunshades, a 3 mm x 27 mm star scope, and easy-to-read micrometer drum vernier scale that reads to 2/10 of a minute. The Mark 15 has a 7″ (18 cm) frame radius graduated from 120 degrees to -5 degrees and comes with a traditional half-silvered mirror.
The MK 25, and MK 15 have the same basic construction. The former is gray in color, and is said to be of a more stable plastic which is resistant to temperature changes. The latter is black in color. The MK 15 has a traditional split mirror, and is non-illuminated while the MK 25 has illumination and a whole horizon mirror. Both sextants come with an instruction booklet, plastic carrying case, neck lanyard, 3x scope. and a sight tube.